Phosphorous acid
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| Phosphorous acid | |
|---|---|
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| General | |
| Systematic name | phosphorous acid |
| Other names | phosphonic acid |
| Molecular formula | H3PO3 |
| SMILES | ? |
| Molar mass | 82.00 g/mol |
| Appearance | colorless solid |
| CAS number | [13598-36-2] |
| Properties | |
| Density and phase | 1.65 g/cm3 |
| Solubility in water | fully miscible |
| Melting point | 70.1 °C |
| Boiling point | decomposes |
| Acidity (pKa1) | 2.0 |
| Acidity (pKa2) | 6.7 |
| Structure | |
| Molecular shape | tetrahedral |
| Crystal structure | ? |
| Dipole moment | ? D |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | skin irritant |
| NFPA 704 | |
| R/S statement | R: 22-35 S: 26-36/37/39-45 |
| RTECS number | SZ6400000 |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Related ? | ? |
| Related compounds | H3PO4 (i.e., PO(OH)3) H3PO2 (i.e., H2PO(OH)) |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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The compound phosphorous acid, with formula H3PO3, is one of the oxoacids of phosphorus. The other important members of this family are phosphoric acid, H3PO4, and hypophosphorous acid, H3PO2. Note that only the reduced phosphorus compounds are spelled with an "ous" ending. Other names for this acid are orthophosphorous acid and dihydroxyphosphine oxide.
HP(O)(OH)2 is the product of the hydrolysis of its acid anhydride, P4O6:
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- P4O6 + 6 H2O → 4 HP(O)(OH)2
An analogous relationship connects H3PO4 and P4O10.
Contents |
[edit] Tautomerization
H3PO3 is better described with the structural formula HP(O)(OH)2. This species exists in equilibrium with a minor tautomer P(OH)3. The latter is called phosphorous acid, whereas the dihydroxy form is called phosphonic acid. Many of the reduced phosphorus acids are subject to similarly complicated equilibria involving shifts of H between O and P. In the solid state, HP(O)(OH)2 is tetrahedral with one shorter P=O bond of 1.48 Å and two longer P-O(H) bonds of 1.54 Å.
[edit] Preparation
Although commercially available, the acid is most commonly prepared by hydrolysis of phosphorus trichloride with water or steam:
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- PCl3 + 3 H2O → HP(O)(OH)2 + 3 HCl
[edit] Acid-base properties
Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that all three hydrogen atoms are not acidic under aqueous conditions, in contrast with phosphoric acid. HP(O)2(OH)- is a moderately strong acid.
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- HP(O)(OH)2 → HP(O)2(OH)- + H+ pKa = 2.0
- HP(O)2(OH)- → HPO32- + H+ pKa = 6.7
The monodeprotonated species, HP(O)2(OH)-. is called the phosphite ion.
The IUPAC (mostly organic) name is phosphonic acid. This is commonly used for a substituted compound (that is, organic group bonded to phosphorus, not simply an ester). For example, (CH3)PO(OH)2 is "methylphosphonic acid", which may of course form "methylphosphonate" esters.
Both phosphorous acid and its deprotonate forms are rather good reducing agents, although not necessarily quick to react. They are oxidized to phosphoric acid or its salts. It reduces solutions of noble metal cations to the metals.
[edit] Uses
[edit] Conversion to phosphine
Phosphine, being a flammable and toxic gas, is inconveniently stored. Fortunately this useful species is readily prepared by thermal decomposition of phosphorous acid:
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- 4 HP(O)(OH)2 → PH3 + 3 H3PO4
Since phosphoric acid is a syrupy non-volatile liquid, the gaseous PH3 is readily separated.
[edit] Use as anti-microbial agent
Phosphorous acid has shown effectiveness in controlling dieback, and is marketed in Australia as 'Anti-Rot', an anti-microbial agent.
[edit] Other uses
A large bulk of this acid is used as phosphatic fertilizer. Pure acid is also used for preparation phosphate salts
[edit] References
- Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
